A study of natural and artificial actuation systems

Abstract

To match with the excellent actuating performance of the skeletal muscle, simple and self-contained actuators that function by different mechanisms have been developed in the last two decades. These actuators are also known as artificial muscles. The present actuation systems, such as elastomers, shape memory alloys, conducting polymers and ionic polymer metal composites have been extensively investigated by different researchers. The performance of these systems has been improving, but still cannot compete with the skeletal muscles which can produce high stress, strain, work density and power density. In the light of this, the present work aims at developing new actuation systems. Two approaches will be described in this thesis. The first was inspired from nature –the fast movement of the plant Mimosa pudica. The second approach was the actuation of nanostructured materials, which have been found to possess different properties from the macro-counterpart.

From the mathematical study on the motor cells of M. pudica, it was found that a ten-fold increase in the ionic permeability of the cell membrane is able to trigger the fast movement, which can produce a decrease in the motor cell diameter of 20 to 38 % in a few seconds. Basically, the increase in the ionic permeability disrupts the balance between the intracellular and extracellular ion concentrations, causing an efflux of ions through diffusion and migration, followed by water efflux by osmosis. The strain in the motor cell diameter demonstrated a comparable strain produced by the skeletal muscle. Therefore, the mechanism for inducing the cell volume change was tested in leukemia cells. A successful control of the cell volume through electrodialysis was demonstrated. Sustainable strains of ±15 %in the cell diameter were obtained by applying an electric potential difference across a cation selective membrane, causing electrodialysis that altered the extracellular osmolarity and hence the cell volume. A mathematical model based on the ion and water transport across the membrane and the change of cell volume was proposed. The consistent results of the model and the experiment show that the mechanism of the strain was indeed the osmolarity change by the electrodialysis. This study has demonstrated a new method for realizing bio-actuation.

For the actuation of nanostructured materials, two thin-film systems made of gold (Au) were introduced. Powered through electric charging in air and an applied voltage under an electrolyte respectively, the two systems could produce the strains of 0.08 % and 8.1×〖10〗^(-4) %. The mechanisms wererespectivelythe electrostatic repulsion and the change in surface stress of the nanostructured Au. Although the actuation strains are relatively small compared to the present artificial muscles, the results demonstrated two novel ways to build actuators by nanostructured materials. Together with the results on bio-actuation, the studies on the actuation of nanostructured materials have led to potential approaches in the development of artificial muscles.